Dietary fiber is what we call the material in food that is neither nutrient nor
water. It is the material that our bodies' cannot digest because we lack the
enzymes required to break it down. By virtue of this, nutritionists consider
most fiber (except 'resistant starches") to be calorie-free.
Insoluble Dietary Fiber
Insoluble fiber is the kind of fiber that we can see. It is the "sawdust-like"
material found covering grains. Wheat bran for example, is nearly all insoluble
fiber. The tough stringy material in leafy vegetables, especially celery, is
another example. Generally, insoluble fiber is composed of cellulose, a
carbohydrate, that is the main structural molecule in plants. Wood gets its
strength from long strands of cellulose that are "glued" together by chemicals
known as lignins. Paper is made by purifying from wood chips all the cellulose
while getting rid of the lignins. When people say that bran tastes "like
cardboard", in fact it almost is.
Be that as it may, insoluble fiber is very important to human health and to
weight control. Insoluble fiber improves bowel regularity and decreases the
time that stool is insisde the colon before elimination which appears to reduce
risk of cancer of the colon. Because insoluble fiber has "bulk" without calories,
it can mechanically fill the stomach for "free" so that a person's desire to stop
eating kicks in sooner. Finally, insoluble fiber delays entry of sugar into the
blood stream after eating so that fiber lowers the "glycemic index" of foods.
Foods that are high in fiber are generally "whole" plant foods; grains that retain
their outer husk, whole fruits (and NOT fruit juices) and whole (but necessarily
raw) vegetables. Interestingly, shellfish and insect carapaces also are
considered fiber, in this case a fiber called "chitin", but generally Americans
don't eat a lot of insects and like to peel their shrimp.
Soluble Dietary Fiber:
Soluble fibers are generally plant-produced carbohydrates that, like cellulose,
are indigestible, but will dissolve in water and therebye form a slippery gel.
Metamucil-the husk of a plant called psyllium-is a good and perhaps familiar
example of soluble dietary fiber. So is fruit pectin, guar gum, xanthan gum and
the fiber in oats that makes oatmeal "gluey" or slimy.
Soluble fibers too are important in human health and weight control and for
largely the same reasons as insoluble fiber.
These are plant produced carbohydrates that the human gut cannot digest
that are somewhat different than other forms of fiber. Resistant starches often
have a starchy or even sweet taste and they are neither totally insoluble nor
prone to forming gels. The main difference however is that resistant starches
are not calorie-free. Generally, they provide us about 2 calories per gram of
resistant starch. How is this possible given that I just stated that resistant
starches are indigestible?
The answer is that resistant starches are indigestible using human gut
enzymes, but, not indegistible to many of the bacteria that inhabit the human
colon. Thus, resistant starches pass totally unscathed all the way through the
small intestine, but once they enter the colon they are digested but bacteria.
However, the bacteria do not totally digest resistant starches; they turn some
of them into fatty acids that the bacteria then secrete and which we then
absorb as nutrient. This is why resistant starches provide a small amount of
More importantly, resistant starches appear to selectively nourish the "good"
kinds of bacteria in the gut and therebye promote regularity and reduce risk for
cancer. Because of their ability to promote beneficial bacteria, resistant
starches are also called "pre-biotic" nutrients.
Resistant starches are often found in roots of plants, but not all roots. A weird
sunflower-like plant called a jerusalem artichoke has roots that are loaded with
a resistant starch called "inulin" (NOT "insulin"). Regular artichokes also have a
fair amount of inulin. A sweetener that has long been popular in Mexico is now
being sold in the US and it is made from the root of the "blue agave" cactus
which is also inulin-rich.
Side Effects of Fiber
People need to ease into a higher fiber diet. Otherwise, a rapid switch from a
low fiber to a high fiber diet can be accompanied by a lot of intestinal gas that
can by quite uncomfortable and will often cause people to abandon fiber.
Generally, a slow increase in dietary fiber over a month or two will not cause
any of these problems.
All forms of fiber benefit health and improve weight loss and therefore, people
should strive to eat fiber rich foods.
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